1. Field of the Invention
The invention relates to a control apparatus and a control method that control deceleration of a vehicle or braking force on the vehicle when the vehicle is decelerated. In particular, the invention relates to a control apparatus and a control method that execute the same control so as to prevent an excessive change in the braking force even upon occurrence of an event that may cause a change in torque, such as releasing of a lockup clutch and discontinuance of fuel-cut control.
2. Description of Related Art
An internal combustion engine that produces drive force through combustion of fuel, such as a gasoline engine, starts running on its own by fuel supply while the internal combustion engine is driven at a certain engine speed or higher. When a vehicle incorporating an internal combustion engine is being decelerated, the drive wheels and the internal combustion engine are connected to each other such that torque is transmittable between them, so that the internal combustion engine is forcibly rotated by the inertial force of the traveling vehicle. Then, in response to the speed of the internal combustion engine becoming equal to or higher than a predetermined speed, the supply of fuel is suspended (“fuel-cut” is performed). Then, the fuel supply is resumed in response to the speed of the internal combustion engine reaching the speed at which to resume the fuel supply (i.e., the speed at which to discontinue the fuel-cut) with a decrease in the vehicle speed. Such control for suspending fuel supply is often called “fuel-cut control”, and it is normally adapted to, in order to suspend the fuel supply as long as possible for further improvement of the fuel economy, increase the transmission speed ratio gradually as the vehicle speed decreases, so as to maintain the speed of the internal combustion engine higher than the engine speed at which to resume the fuel supply while preventing the braking force from becoming excessive.
Torque converters are widely used to facilitate the control for starting the vehicle and the control for shifting the transmission. Most torque converters are provided with a lockup clutch that mechanically, and directly, connect the pump impeller and the turbine runner with each other in order to minimize a decrease in the drive force transmission efficiency, which is caused by relative rotations of the pump impeller and turbine runner (often called “slipping”). As long as the lockup clutch is applied, the connection between the drive wheels and the internal combustion engine is more “direct”, making it easier to maintain the speed of the internal combustion engine at a desired speed or in a desired speed range. For this reason, normally, the lockup clutch is applied during the fuel-cut control.
However, a change in torque may occur as a result of the lockup clutch being applied, causing vibrations that propagate to the vehicle body and then are felt by the occupants including the driver, and further such vibrations increase booming noises, degrading the ride comfort. To counter this, normally, the lockup clutch is applied on the condition that the vehicle speed is equal to or higher than a predetermined speed. Thus, the lock-up clutch is released when the vehicle speed has been reduced to the predetermined speed in response to the driver's deceleration operation or under given deceleration control. However, this results in a decrease in the capacity of torque transmission between the internal combustion engine and the drive wheels, thus, a decrease in the braking force obtained from the internal combustion engine. Such a change in the braking force may act as “shock”. Japanese Patent Application Publication No. 2002-234340 describes an invention designed to reduce such a shock occurring as a result of the lockup clutch being released during deceleration of the vehicle.
More specifically, Japanese Patent Application Publication No. 2002-234340 describes an apparatus that is structured to extend the duration of fuel-cut control for suspending the fuel supply to the internal combustion engine, by maintaining the engine speed at a relatively high speed by applying the lockup clutch during deceleration of the vehicle. This control is aimed at maintaining the engine speed higher than the speed at which to resume the fuel supply. To this end, the apparatus described in Japanese Patent Application Publication No. 2002-234340 reduces, during the fuel-cut control with the lockup clutch applied, the load of the air conditioner to minimize a decrease in the engine speed. According to the apparatus described in Japanese Patent Application Publication No. 2002-234340, further, the vehicle speed at which to release the lockup clutch is set such that the decrease in deceleration, which occurs as a result of the lockup clutch being released, does not cause uncomfortable feelings.
As mentioned above, the apparatus described in Japanese Patent Application Publication No. 2002-234340 sets the lockup clutch release vehicle speed such that the deceleration does not cause uncomfortable feelings. Thus, the lockup clutch release vehicle speed is relatively high, and this may result in a shorter duration of the fuel-cut control, ending up reduced fuel economy improvement. Further, in a case where an accessory component mounted on the engine is used to control the braking force, if the torque of the accessory component or the control of the same torque is restricted for some reason, the braking torque may fail to be controlled as required, leaving a possibility of occurrence of “shock”. Sufficient study has not been conducted on control for coping with such a situation, and thus it is necessary to develop a new technique.